The systems provided for carrying out the “Sense & Avoid” function on board drones use conventional radars operating in the millimeter band, for example in the Ku or Ka bands, which are derived from radars provided for other functions. The current state of regulations does not permit the flight of automatic craft within the general air traffic.
These conventional systems are equipped with a rotary mechanical antenna. They have a naturally slender beam because of their high operating frequency. They thus allow precise tracking, on the other hand, their scan speed must be very high to cover an angular domain suited to a function of “Sense & Avoid” type at a sufficient rate. Typically, such a function ought to cover an angular field comparable to a pilot's angle of vision, i.e. about ±110° in azimuth and 40° in elevation. The angular coverage constraint is therefore only partially satisfied. Moreover, these conventional systems require a protuberance of the drones since they cannot be placed directly on their structure, thus posing aerodynamism or bulkiness problems.
Conventional electronic scanning, using a slender formed beam, can solve this fast coverage problem, but it exhibits the drawbacks of the complexity of the time allocated to the processing of a given direction, which becomes extremely small on account of the angular field to be covered in a given timescale. One solution for solving these problems is to use a computational beamforming system, termed CBF. Indeed, a CBF system makes it possible to carry out continuous observation of a plurality of direction by means of slender beams formed simultaneously by computation, and makes it possible to overlay one or more antenna systems directly onto the structure of a drone, or of any other aircraft, without significant additional bulkiness, since it is no longer necessary to provide for antenna rotation.
An exemplary simple CBF system operates in the following manner:                a certain number of CBF antenna systems are distributed over an aircraft so as to cover the whole of the domain to be monitored, each of the systems being composed:                    of a wide-field emission antenna associated with an emission system placed as close as possible so as to minimize losses, in practice the maximum coverage of a wide-field antenna overlaid on an aircraft structure being at the maximum about ±60°, at least two systems then being necessary in order to cover the above-mentioned domain;            of N reception antennas placed in an array, each having, at least, the same individual angular coverage as the emission antenna, each of these antennas being associated, as closely as possible, directly with a receiver so as to minimize losses, thus making it possible to obtain low loss at reception;                        
The range R of a radar monitoring a certain solid angle is given by the following proportionality relation:
                              R          4                ∝                                            P              E                        ⁢                          TA              R                        ⁢                          σ              ⁡                              (                λ                )                                                                        L              ·              Ω                        ⁢                                                                                    (        1        )            where PE represents the power emitted, AR the reception antenna surface area, λ the wavelength, σ(λ) the equivalent radar cross section of the target, denoted RCS, which may be dependent on the length λ, T the repetition period of surveillance of the domain of solid angle Ω and L the microwave frequency losses. This relation assumes that the emission antenna pattern exactly covers the domain Ω. This condition fixes the gain of the emission antenna.
It is apparent that if the RCS of the target is independent of wavelength, so also is the surveillance range.
However, this simple solution exhibits a drawback as soon as a tracking and precise angular location function is necessary. Indeed:                if the chosen frequency band is low, for example the L band, the number of modules necessary to cover the reception area AR is small, therefore the cost low, but the lobes formed by computation are wide, therefore hardly favourable to angular precision;        if the chosen frequency band is high, for example the X band and beyond, the number of modules necessary to cover the reception area AR is high and the cost becomes prohibitive, moreover the distance-speed ambiguity conditions are hardly favourable to the X band, on the other hand the lobes formed by computation are slender and therefore favourable to angular precision.        
A good compromise is to work in the S band. Moreover the constraints related to spectral congestion are less critical than in other neighbouring bands. The choice of this band is more favourable than the L band notably from an angular precision point of view, but the location precision problem remains. Indeed, the coverage of the emission antenna is well suited to the search domain but is unsuited to the domain necessary when tracking. When tracking, the majority of the energy emitted is lost in directions where the target cannot be present since its position is then known a priori. This results in a needless loss of the signal-to-noise ratio, S/N, in relation to conventional radar systems.